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        <p>Library Version 12.1.6.2</p>
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          <th colspan="3" align="center">Example Processing Loop</th>
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          <td width="20%" align="left"><a accesskey="p" href="processingloop.html">Prev</a> </td>
          <th width="60%" align="center">Chapter 4. Replica versus Master Processes</th>
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    <div class="sect1" lang="en" xml:lang="en">
      <div class="titlepage">
        <div>
          <div>
            <h2 class="title" style="clear: both"><a id="exampledoloop"></a>Example Processing Loop</h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <dl>
          <dt>
            <span class="sect2">
              <a href="exampledoloop.html#runningit">Running It</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>
                          In this section we take the example
                          processing loop that we presented in the
                          previous section and we flesh it out to
                          provide a more complete example. We do this
                          by updating the
                          <code class="function">doloop()</code>
                          function that our original transaction
                          application used 
                          <span>(see <a class="xref" href="simpleprogramlisting.html#doloop_c" title="Function: doloop()">Function: doloop()</a>)</span>
                          
                          
                          to fully support our replicated application. 
                  </p>
      <p>
                          In the following example code, code that we
                          add to the original example is presented in 
                          <strong class="userinput"><code>bold</code></strong>.
                  </p>
      <p>
                          To begin, we include a new header file into
                          our application so that we can check for the 
                          <code class="literal">ENOENT</code> return value later
                          in our processing loop. We also define our 
                            <code class="literal">APP_DATA</code>
                          structure, and we define a
                          <code class="literal">sleeptime</code> value. 

                          <span>
                          Finally, we
                          add a new forward declaration for our event
                          callback.
                          </span>

                          
                  </p>
      <pre class="programlisting">/*
 * File: rep_mgr_gsg.c
 */

#include &lt;stdlib.h&gt;
#include &lt;string.h&gt;
<strong class="userinput"><code>#include &lt;errno.h&gt;</code></strong>
#ifndef _WIN32
#include &lt;unistd.h&gt;
#endif

#include &lt;db.h&gt;

#ifdef _WIN32
extern int getopt(int, char * const *, const char *);
#endif


#define CACHESIZE   (10 * 1024 * 1024)
#define DATABASE    "quote.db"
<strong class="userinput"><code>#define SLEEPTIME 3</code></strong>

const char *progname = "ex_rep_gsg_repmgr";

<strong class="userinput"><code>typedef struct {
    int is_master;
} APP_DATA;</code></strong>

int create_env(const char *, DB_ENV **);
int env_init(DB_ENV *, const char *);
int doloop (DB_ENV *);
static int print_stocks(DB *);
<strong class="userinput"><code>void *event_callback(DB_ENV *, u_int32_t, void *);</code></strong> </pre>
      <p>
        In our <code class="function">main()</code> function, most of what we
        have to add to it is some new variable declarations and
        initializations:
</p>
      <pre class="programlisting">int
main(int argc, char *argv[])
{
    DB_ENV *dbenv;
    DB_SITE *dbsite; 
    extern char *optarg;
    const char *home;
    char ch, *host, *portstr;
    int ret, local_is_set, is_group_creator;
    u_int32_t port;
    <strong class="userinput"><code>/* Used to track whether this is a replica or a master */
    APP_DATA my_app_data;

    my_app_data.is_master = 0;  /* Assume that we start as a replica */</code></strong>
    dbenv = NULL;

    ret = local_is_set = is_group_creator = 0;
    home = NULL;</pre>
      <p>
                The rest of our <code class="function">main()</code> function
                is unchanged, except that we make our
                <code class="literal">APP_DATA</code> structure available through our
                environment handle's <code class="methodname">app_private</code>
                field:
        </p>
      <pre class="programlisting">
    if ((ret = create_env(progname, &amp;dbenv)) != 0)
            goto err;

    <strong class="userinput"><code>/* Make APP_DATA available through the environment handle */
    dbenv-&gt;app_private = &amp;my_app_data;</code></strong>

    /* Default priority is 100 */
    dbenv-&gt;rep_set_priority(dbenv, 100);
    /* Permanent messages require at least one ack */
    dbenv-&gt;repmgr_set_ack_policy(dbenv, DB_REPMGR_ACKS_ONE);
    /* Give 500 microseconds to receive the ack */
    dbenv-&gt;rep_set_timeout(dbenv, DB_REP_ACK_TIMEOUT, 500);

    while ((ch = getopt(argc, argv, "h:l:L:p:r:")) != EOF)
        switch (ch) {
        case 'h':
            home = optarg;
            break;
        /* Set the host and port used by this environment */
        case 'L':
            is_group_creator = 1; /* FALLTHROUGH */
        case 'l':
            host = strtok(optarg, ":");
            if ((portstr = strtok(NULL, ":")) == NULL) {
                fprintf(stderr, "Bad host specification.\n");
                goto err;
            }
            port = (unsigned short)atoi(portstr);
            if ((ret = dbenv-&gt;repmgr_site(dbenv, host, port, &amp;dbsite
                                          0)) != 0 ) {
                fprintf(stderr,
                    "Could not set local address %s.\n", host);
                goto err;
            }
            dbsite-&gt;set_config(dbsite, DB_LOCAL_SITE, 1);
            if (is_group_creator)
                dbsite-&gt;set_config(dbsite, DB_GROUP_CREATOR, 1);

            if ((ret = dbsite-&gt;close(dbsite)) != 0) {
                dbenv-&gt;err(dbenv, ret, "DB_SITE-&gt;close");
                goto err;
            }
            local_is_set = 1;
            break;
        /* Set this replica's election priority */
        case 'p':
            dbenv-&gt;rep_set_priority(dbenv, atoi(optarg));
            break;
        /* Identify another site in the replication group */
        case 'r':
            host = strtok(optarg, ":");
            if ((portstr = strtok(NULL, ":")) == NULL) {
                fprintf(stderr, "Bad host specification.\n");
                goto err;
            }
            port = (unsigned short)atoi(portstr);
            if ((dbenv-&gt;repmgr_site(dbenv, host, port, &amp;dbsite,
                                      0)) != 0) {
                dbenv-&gt;err(dbenv, ret, "DB_ENV-&gt;repmgr_site");
                goto err;
            }
            dbenv-&gt;set_config(dbsite, DB_BOOTSTRAP_HELPER, 1);
            if ((dbenv-&gt;close(dbsite)) != 0) {
                dbenv-&gt;err(dbenv, ret, "DB_SITE-&gt;close");
                goto err;
            }
            break;
        case '?':
        default:
            usage();
        }

    /* Error check command line. */
    if (home == NULL || !local_is_set)
        usage();

    if ((ret = env_init(dbenv, home)) != 0)
            goto err;

    if ((ret = dbenv-&gt;repmgr_start(dbenv, 3, DB_REP_ELECTION)) != 0)
        goto err;

    /* Sleep to give ourselves time to find a master. */
    sleep(5);

    if ((ret = doloop(dbenv)) != 0) {
        dbenv-&gt;err(dbenv, ret, "Application failed");
        goto err;
    }

err: if (dbenv != NULL)
        (void)dbenv-&gt;close(dbenv, 0);

    return (ret);
} </pre>
      <p>
            Having updated our <code class="function">main()</code>, we must also
            update our <code class="function">create_env()</code> function to
            register our <code class="function">event_callback</code> callback.
            Notice that our <code class="function">env_init()</code> function, which is
            responsible for actually opening our environment handle, is
            unchanged:
    </p>
      <pre class="programlisting">int
create_env(const char *progname, DB_ENV **dbenvp)
{
    DB_ENV *dbenv;
    int ret;

    if ((ret = db_env_create(&amp;dbenv, 0)) != 0) {
        fprintf(stderr, "can't create env handle: %s\n",
            db_strerror(ret));
        return (ret);
    }

    dbenv-&gt;set_errfile(dbenv, stderr);
    dbenv-&gt;set_errpfx(dbenv, progname);
    <strong class="userinput"><code>(void)dbenv-&gt;set_event_notify(dbenv, event_callback);</code></strong>

    *dbenvp = dbenv;
    return (0);
} 

int
env_init(DB_ENV *dbenv, const char *home)
{
    u_int32_t flags;
    int ret;

    (void)dbenv-&gt;set_cachesize(dbenv, 0, CACHESIZE, 0);
    (void)dbenv-&gt;set_flags(dbenv, DB_TXN_NOSYNC, 1);

    flags = DB_CREATE |
            DB_INIT_LOCK |
            DB_INIT_LOG |
            DB_INIT_MPOOL |
            DB_INIT_REP |
            DB_INIT_TXN |
            DB_RECOVER |
            DB_THREAD;
    if ((ret = dbenv-&gt;open(dbenv, home, flags, 0)) != 0)
        dbenv-&gt;err(dbenv, ret, "can't open environment");
    return (ret);
}</pre>
      <p>
        That done, we need to implement our
        <code class="function">event_callback()</code> callback. Note that what we use
        here is no different from the callback that we described in
        the previous section. However, for the sake of completeness we
        provide the implementation here again.
</p>
      <pre class="programlisting">
        <strong class="userinput">
          <code>/*
 * A callback used to determine whether the local environment is a 
 * replica or a master. This is called by the Replication Manager
 * when the local replication environment changes state.
 */
void *
event_callback(DB_ENV *dbenv, u_int32_t which, void *info)
{
    APP_DATA *app = dbenv-&gt;app_private;

    info = NULL;                /* Currently unused. */

    switch (which) {
    case DB_EVENT_REP_MASTER:
        app-&gt;is_master = 1;
        break;

    case DB_EVENT_REP_CLIENT:
        app-&gt;is_master = 0;
        break;

    case DB_EVENT_REP_STARTUPDONE: /* fallthrough */
    case DB_EVENT_REP_NEWMASTER:
        /* Ignore. */
        break;

    default:
        dbenv-&gt;errx(dbenv, "ignoring event %d", which);
    }
}</code>
        </strong>
      </pre>
      <p>
        That done, we need to update our
        <code class="function">doloop()</code> 
        <span>function. This is the place where we most
        heavily modify our application.</span>

        
</p>
      <p>
        We begin by introducing <code class="literal">APP_DATA</code> to the
        function:
</p>
      <pre class="programlisting">/*
 * Provides the main data processing function for our application.
 * This function provides a command line prompt to which the user
 * can provide a ticker string and a stock price. Once a value is
 * entered to the application, the application writes the value to
 * the database and then displays the entire database.
 */
#define BUFSIZE 1024
int
doloop(DB_ENV *dbenv)
{
    DB *dbp;
    <strong class="userinput"><code>APP_DATA *app_data;</code></strong>
    DBT key, data;
    char buf[BUFSIZE], *rbuf;
    int ret;
    u_int32_t flags;

    dbp = NULL;
    ret = 0;
    memset(&amp;key, 0, sizeof(key));
    memset(&amp;data, 0, sizeof(data));
    <strong class="userinput"><code>app_data = dbenv-&gt;app_private;</code></strong></pre>
      <p>
        Next we begin to modify our main loop. 
        To start, upon entering the loop we create the database handle and configure it as
        normal. But we also have to decide what flags we will use for
        the open. Again, it depends on whether we are a replica or a
        master.
    </p>
      <pre class="programlisting">    for (;;) {
        if (dbp == NULL) {
            if ((ret = db_create(&amp;dbp, dbenv, 0)) != 0)
                return (ret);

            <strong class="userinput"><code>flags = DB_AUTO_COMMIT;
            if (app_data-&gt;is_master)
                flags |= DB_CREATE;</code></strong> </pre>
      <p>
    When we open the database, we modify our error handling to
    account for the case where the database does not yet exist. This can
    happen if our code is running as a replica and the Replication Manager has not
    yet had a chance to create the databases for us. Recall that replicas never
    write to their own databases directly, and so they cannot
    create databases on their own.
</p>
      <p>
        If we detect that the database does not yet exist, we simply
        close the database handle, sleep for a short period of time
        and then continue processing. This gives the Replication Manager a chance to
        create the database so that our replica can continue
        operations.
</p>
      <pre class="programlisting">            if ((ret = dbp-&gt;open(dbp,
                NULL, DATABASE, NULL, DB_BTREE, flags, 0)) != 0) {
                <strong class="userinput"><code>if (ret == ENOENT) {
                    printf(
                      "No stock database yet available.\n");
                    if ((ret = dbp-&gt;close(dbp, 0)) != 0) {
                        dbenv-&gt;err(dbenv, ret,
                            "DB-&gt;close");
                        goto err;
                    }
                    dbp = NULL;
                    sleep(SLEEPTIME);
                    continue;
                }</code></strong>
                dbenv-&gt;err(dbenv, ret, "DB-&gt;open");
                goto err;
            }
        } </pre>
      <p>
        Next we modify our prompt, so that if the local process is running
        as a replica, we can tell from the shell that the prompt is for a
        read-only process.
    </p>
      <pre class="programlisting">        printf("QUOTESERVER<strong class="userinput"><code>%s</code></strong>&gt; ",
            <strong class="userinput"><code>app_data-&gt;is_master ? "" : " (read-only)");</code></strong>
        fflush(stdout); </pre>
      <p>
            When we collect data from the prompt, there is a case that says
            if no data is entered then show the entire stocks database.
            This display is performed by our
            <code class="function">print_stocks()</code> 
            <span>function</span> 
             
            (which has not
            required a modification since we first introduced it in 
            <a class="xref" href="simpleprogramlisting.html#printstocks_c" title="Function: print_stocks()">
                            <span>Function: print_stocks()</span>
                            
                            
                    </a>).
    </p>
      <p>
            When we call 
               <span><code class="function">print_stocks()</code>, </span>
               
            we check for a dead replication handle. Dead
            replication handles happen whenever a replication election
            results in a previously committed transaction becoming
            invalid. This is an error scenario caused by a new master having a
            slightly older version of the data than the original
            master and so all replicas must modify their database(s) to
            reflect that of the new master. In this situation, some
            number of previously committed transactions may have to be
            unrolled. From the replica's perspective, the database
            handles should all be closed and then opened again.
    </p>
      <pre class="programlisting">        if (fgets(buf, sizeof(buf), stdin) == NULL)
            break;
        if (strtok(&amp;buf[0], " \t\n") == NULL) {
            switch ((ret = print_stocks(dbp))) {
            case 0:
                continue;
            <strong class="userinput"><code>case DB_REP_HANDLE_DEAD:
                (void)dbp-&gt;close(dbp, DB_NOSYNC);
                dbp = NULL;
                dbenv-&gt;errx(dbenv, "Got a dead replication handle");
                continue; </code></strong>
            default:
                dbp-&gt;err(dbp, ret, "Error traversing data");
                goto err;
            }
        }
        rbuf = strtok(NULL, " \t\n");
        if (rbuf == NULL || rbuf[0] == '\0') {
            if (strncmp(buf, "exit", 4) == 0 ||
                strncmp(buf, "quit", 4) == 0)
                break;
            dbenv-&gt;errx(dbenv, "Format: TICKER VALUE");
            continue;
        }</pre>
      <p>
            That done, we need to add a little error checking to our
            command prompt to make sure the user is not attempting to
            modify the database at a replica. Remember, replicas must never
            modify their local databases on their own. This guards against
            that happening due to user input at the prompt.
    </p>
      <pre class="programlisting">        <strong class="userinput"><code>if (!app_data-&gt;is_master) {
            dbenv-&gt;errx(dbenv, "Can't update at client");
            continue;
        }</code></strong>
        key.data = buf;
        key.size = (u_int32_t)strlen(buf);

        data.data = rbuf;
        data.size = (u_int32_t)strlen(rbuf);

        if ((ret = dbp-&gt;put(dbp,
            NULL, &amp;key, &amp;data, 0)) != 0) {
            dbp-&gt;err(dbp, ret, "DB-&gt;put");
            goto err;
        }
    }

err:    if (dbp != NULL)
        (void)dbp-&gt;close(dbp, DB_NOSYNC);

    return (ret);
} </pre>
      <p>
        With that completed, we are all done updating our application
        for replication. 

        The only remaining 
        <span>function, <code class="function">print_stocks()</code>,</span> 
         
         

        is unmodified from when we
        originally introduced it. For details on that function, see
        <a class="xref" href="simpleprogramlisting.html#printstocks_c" title="Function: print_stocks()">
                            <span>Function: print_stocks()</span>
                            
                            
                    </a>.
</p>
      <div class="sect2" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h3 class="title"><a id="runningit"></a>Running It</h3>
            </div>
          </div>
        </div>
        <p>
                To run our replicated application, we need to make
                sure each participating environment has its own unique
                home directory. We can do this by running
                each site on a separate networked machine, but that
                is not strictly necessary; multiple instances of this
                code can run on the same machine provided the
                environment home restriction is observed.
        </p>
        <p>
                To run a process, make sure the environment home exists and
                then start the process using the <code class="literal">-h</code>
                option to specify that directory. You must also use the
                <code class="literal">-l</code> or  <code class="literal">-L</code> option to
                identify the local host and port that this process will use
                to listen for replication messages (-L means that this is a
                group creator), and the <code class="literal">-r</code> option to
                identify the other processes in the replication group.
                Finally, use the <code class="literal">-p</code> option to specify a
                priority.  The process that you designate to have the
                highest priority will become the master.
        </p>
        <pre class="programlisting">&gt; mkdir env1
&gt; ./ex_rep_gsg_repmgr -h env1 -L localhost:8080 -p 10
No stock database yet available.
No stock database yet available.  </pre>
        <p>
        Now, start another process. This time, change the environment
        home to something else, use the <code class="literal">-l</code> flag to at
        least change the port number the process is listening on, and
        use the <code class="literal">-r</code> option to identify the host and
        port of the other replication process:
</p>
        <pre class="programlisting">&gt; mkdir env2
&gt; ./ex_rep_gsg_repmgr -h env2 -l localhost:8081 \
-r localhost:8080 -p 20</pre>
        <p>
        After a short pause, the second process should display the master
        prompt:
</p>
        <pre class="programlisting">
QUOTESERVER &gt; </pre>
        <p>
        And the first process should 
        display the read-only prompt:
</p>
        <pre class="programlisting">
QUOTESERVER (read-only)&gt; </pre>
        <p>
        Now go to the master process and give it a couple of stocks and stock
        prices:
</p>
        <pre class="programlisting">QUOTESERVER&gt; FAKECO 9.87
QUOTESERVER&gt; NOINC .23
QUOTESERVER&gt; </pre>
        <p>
    Then, go to the replica and hit <strong class="userinput"><code>return</code></strong> at the prompt to
    see the new values:
</p>
        <pre class="programlisting">QUOTESERVER (read-only)&gt; 
        Symbol  Price
        ======  =====
        FAKECO  9.87
        NOINC    .23 
QUOTESERVER (read-only)&gt; </pre>
        <p>
        Doing the same at the master results in the same thing:
</p>
        <pre class="programlisting">QUOTESERVER&gt; 
        Symbol  Price
        ======  =====
        FAKECO  9.87
        NOINC    .23 
QUOTESERVER&gt; </pre>
        <p>
        You can change a stock by simply entering the stock value and
        new price at the master's prompt:
</p>
        <pre class="programlisting">QUOTESERVER&gt; FAKECO 10.01 
QUOTESERVER&gt; </pre>
        <p>
        Then, go to either the master or the replica to see the updated
        database. On the master:
</p>
        <pre class="programlisting">QUOTESERVER&gt; 
        Symbol  Price
        ======  =====
        FAKECO  10.01
        NOINC    .23 
QUOTESERVER&gt; </pre>
        <p>
        And on the replica:
</p>
        <pre class="programlisting">QUOTESERVER (read-only)&gt; 
        Symbol  Price
        ======  =====
        FAKECO  10.01
        NOINC    .23 
QUOTESERVER (read-only)&gt; </pre>
        <p>
        Finally, to quit the applications, simply type
        <code class="literal">quit</code> at both prompts. On the replica:
</p>
        <pre class="programlisting">QUOTESERVER (read-only)&gt; quit
&gt; </pre>
        <p>
        And on the master as well:
</p>
        <pre class="programlisting">QUOTESERVER&gt; quit
&gt; </pre>
      </div>
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